A helpless neighbour has described watching in horror as an elderly woman was killed by an alligator while walking her dog in a retirement community in the southern US state of Florida.

The neighbour, identified only as Carol, said she saw the woman, who has not been named, being dragged into the water by the 10-foot (three-meter) reptile during an attack at the Spanish Lakes Fairways community in Fort Pierce on Monday.

"(I) just remember her coming up and you know... getting air, and I'm saying swim toward the, swim toward the paddle boat, and she says, 'I can't, the gator has me,'" the woman told local TV station WPBF.

Carol, 77, called emergency services and grabbed a pole in hopes of helping her friend, the station reported. "I thought, well I'll put that out in the water and hook her or hit him, and -- she was not there anymore."

Helpless, Carol could only wait. "I couldn't do anything. I couldn't get in the water." The dog survived the attack, but the woman succumbed to her injuries, the Florida Fish and Wildlife Conservation Commission (FWC) told US media.

A helicopter from the St Lucie County Sheriff's Office helped locate the gator in the lake, WPBF reported, and trappers later dragged it out. "He fought pretty good. Definitely fought pretty good," Nuisance Alligator Trapper Robert Lilly told WPBF.

Lilly said the creature weighed between 600 and 700 pounds (270 to 320 kilograms). "Our thoughts and deepest sympathies are with the family and friends of the victim," the FWC said in a statement, according to US media.

Alligators are commonly seen in Florida, crossing streets, sunning themselves on golf courses or lurking in its extensive waterways. But serious injuries caused by them are rare, with the FWC website saying the likelihood is just one in 3.1 million for a Florida resident. From 1948 to 2021 just 26 bites resulted in human fatalities.

The victim was a widow, Carol said, describing her as "quiet and kind." "She had many good friends here, and she loved her dog," she told WPBF. "I never woke up this Monday morning and thought I would be watching someone die."